FIX: Coverity 167418 Deleting void pointer

Author: jobehrens

libraries/mne/c/mne_msh_display_surface.cpp

@@ -142,8 +142,8 @@ MneMshDisplaySurface::~MneMshDisplaySurface()
     FREE_44(subj);
     FREE_44(surf_name);
     FREE_44(picked);
-    if (user_data_free)
-      delete user_data;
+    if (user_data_free && user_data)
+        (*user_data_free)(user_data);
 
     for (int k = 0; k < nmap; k++)
       delete maps[k];